Use of a tiO2 composition as catalyst for hydrolyzing cos and/or hcn

ABSTRACT

The invention concerns the use of a composition based on TiO 2  as a catalyst for hydrolyzing COS and/or HCN in a gas mixture; said composition comprising at least 1% by weight of at least one sulphate of an alkaline-earth metal selected from calcium, barium, strontium and magnesium.

[0001] The invention relates to the field of catalysts. More precisely,it concerns the use of catalysts intended to encourage hydrolysis ofcarbon oxysulphide (COS) and hydrocyanic acid (HCN) in gas mixturesemanating primarily from co-generation installations.

[0002] It will be recalled that co-generation is a technique for thesimultaneous production of electricity and useful heat (in the form ofsteam or combustion gas) from a fuel such as natural gas, wood, etc.This field is constantly growing. The majority of co-generation unitsare used in installations for the production of electricity.

[0003] The gas from a co-generation installation must satisfy veryparticular specifications linked to the demands of the downstreamprocesses. COS and/or HCN are constituents that are often encounteredand which must be eliminated effectively, for example using a catalyticroute.

[0004] During such transformations, however, the problem must not beexacerbated by extraneous secondary reactions. The CO shift conversionreaction:

CO+H₂O→CO₂+H₂  (1)

[0005] must in particular be avoided, as it suffers from the majorhandicap of reducing the calorific value of synthesis gas because of theinduced rise in the concentration of CO₂ present. A further problem withthat reaction (1) is its exothermic nature, which also increases thetemperature of the medium.

[0006] Further, the catalyst employed to eliminate COS and/or HCN mustadvantageously not result in the formation of formic acid (HCCOH), whichwould contaminate the gases present and would also cause acceleratedageing of the catalyst, and thus would reduce its efficiency and servicelife.

[0007] Other side reactions that should also be avoided are thoseleading to the formation of mercaptans (2), and also of COS from H₂S(3).

CO+H₂S+2H₂→CH₃SH+H₂O  (2)

CO+H₂S→COS+H₂  (3)

[0008] In the specific case in which heavy oil residues are used, tracesof carbonyl metals such as Fe(CO)₅ or Ni(CO)₄ are encountered. Aneffective catalyst for the hydrolysis of COS and HCN must preferably beinert towards those organometallic complexes, so that it is not poisonedduring use under those circumstances.

[0009] A COS and HCN hydrolysis catalyst must also retain its qualitiesin the presence of ammonia and hydrochloric acid, which can also beencountered in the gas to be treated.

[0010] Finally, care should be taken that the catalyst to be used is notitself toxic to human and environmental health.

[0011] Typically, the gas to be treated has concentrations of H₂, CO,H₂S and H₂O in the range 10% to 40%, 15% to 70%, 200 ppm to 3% and 0.5%to 25% respectively. The COS content is normally in the range from 20 to3000 ppm, and that of HCN can reach 1000 ppm. Respective concentrationsof NH₃ and HCl in the range 0 to 2% and in the range 0 to 500 ppm havebeen encountered. All of the concentrations cited above and which willbe cited below are expressed by volume. COS and HCN conversion generallyrequires a temperature in the range 100° C. to 280° C. and a pressurethat can be beyond 60 bars.

[0012] Different COS or HCN hydrolysis catalysts can be found in theliterature. K/alumina, CoMo/alumina, NiMo/alumina and Cr/TiO₂ typeformulations are known. However, their performance is generally mediocrein the case of joint hydrolysis of COS and HCN, and give rise to a highlevel of CO shift conversion. Alumina-based catalysts also induce formicacid formation reactions, and even mercaptan formation. Metal carbonyldecomposition is also observed in all prior art catalysts. Finally,certain of those catalysts, for example those doped with chromium, causeacute problems as regards human and environmental health.

[0013] The aim of the invention is to propose COS and HCN hydrolysiscatalysts that can be used in co-generation installations, which havehigh efficiency and which are free of the disadvantages cited above.

[0014] To this end, the invention concerns the use of a compositionbased on TiO₂ as a catalyst for hydrolyzing COS and/or HCN in a gasmixture, said composition comprising at least 1% by weight, preferablyat least 5%, of at least one sulphate of an alkaline-earth metalselected from calcium, barium, strontium and magnesium.

[0015] In a preferred implementation of the invention, said compositioncomprises at least 40% by weight of TiO₂, preferably at least 60%.

[0016] Said sulphate is preferably calcium sulphate.

[0017] Preferably, the composition also comprises at least one compoundselected from clays, silicates, titanium sulphate and ceramic fibres ina total content of 30% by weight or less, preferably in the range 0.5%to 15%.

[0018] Preferably, said composition comprises at least 60% by weight ofTiO₂, at least 0.1% by weight and at most 20% by weight, advantageouslyat most 15%, preferably at most 10%, of a doping compound or acombination of doping compounds selected from compounds of iron,vanadium, cobalt, nickel, copper, molybdenum and tungsten.

[0019] The doping compound or compounds is/are preferably oxides.

[0020] Preferably, said catalyst has been formed by extrusion.

[0021] Its transverse section can, for example, be in the range 0.5 to 8mm, preferably in the range 0.8 to 5 mm.

[0022] In a preferred application of the invention, the gas mixturederives from a co-generation installation.

[0023] As will become clear, the invention consists of using acomposition based on titanium oxide and containing at least onealkaline-earth metal sulphate, possibly also other compounds, as acatalyst to assist COS and HCN hydrolysis reactions, in particular in aco-generation installation. At the same time, the other side reactionsof formic acid formation, the generation of mercaptans and decompositionof carbonyl metals are advantageously limited compared with thoseobserved with prior art catalysts in this type of application.

[0024] In accordance with the invention, a first principal component ofthe product for use as a catalyst is titanium oxide TiO₂. The otherprincipal component is an alkaline-earth metal sulphate selected fromthe group formed by calcium, barium, strontium and magnesium. Thefunction of said sulphate is to produce a better compromise between thedesired conversions and minimizing side reactions.

[0025] Advantageously, the titanium oxide represents at least 40% of thecomposition weight, preferably at least 60%.

[0026] The preferred alkaline-earth sulphate is calcium sulphate.

[0027] The minimum amount of alkaline-earth sulphate in the compositionis 1% by weight, preferably 5%.

[0028] In addition to titanium oxide and alkaline-earth sulphate, thecomposition can also comprise at least one compound selected from clays,silicates, titanium sulphate and ceramic fibres. The total amount of thecompound or compounds does not exceed 30% by weight, and is preferablyin the range 0.5% to 15%.

[0029] In a particularly-advantageous variation of the invention, thecomposition comprises:

[0030] at least 60% by weight of titanium oxide;

[0031] at least 5% by weight of alkaline-earth sulphate;

[0032] at least 0.1% and at most 20% by weight, advantageously at most15%, and preferably at most 10% of a doping compound or a combination ofdoping compounds selected from compounds of iron, vanadium, cobalt,nickel, copper, molybdenum and tungsten, for example in the form ofoxides.

[0033] The dopant(s) can be added when the titanium oxide andalkaline-earth sulphate are being formed, or subsequent to thatoperation. In the latter case, dry impregnation of one or more solutionsof metal salts is preferable, preparation being completed in aconventional manner by a thermal operation.

[0034] The catalyst can be in any known form: powder, beads, extrudates,monoliths, crushed material, etc. The preferred form in the case of theinvention is the extrudate, either cylindrical or polylobed. Whenforming by mixing followed by extrusion, the transverse section isadvantageously in the range 0.5 to 8 mm, preferably in the range 0.8 to5 mm.

[0035] We shall now describe different examples of compositions for usein the invention, their preparation processes and their properties inthe case of the envisaged use, namely as a catalyst to carry out COS andHCN hydrolysis, in gas mixtures based on CO and H₂ typically comprisingsteam, COS, H₂S and possibly HCN, NH₃ and HCl.

[0036] Three catalysts with compositions in accordance with theinvention, named A, B and C, were produced using the procedure below.

[0037] A suspension of lime was added to a suspension of titanium oxideobtained by hydrolysis and filtration in a conventional ilmenitesulphuric acid attack process, to neutralize all of the sulphatespresent. Once completed, the suspension was dried at 150° C. for onehour. The powder was mixed in the presence of water and nitric acid. Thepaste generated was extruded through a die to obtain extrudates with acylindrical shape. After drying at 120° C. and calcining at 450° C., thediameter of the extrudates was 3.5 mm, and the specific surface area was116 m² μg and a total pore volume of 36 ml/100 g. The TiO₂ content was88% and the CaSO₄ content was 11%; the loss on ignition made the balanceup to 100%. This catalyst was termed A.

[0038] Catalyst B resulted from dry impregnation of an aqueous nickelnitrate solution onto A, followed by drying at 120° C. and calcining at350° C. B then had a nickel content (expressed as NiO) of 2.1% byweight.

[0039] Catalyst C resulted from dry impregnation of an aqueous coppernitrate solution onto A, followed by drying at 120° C. and calcining at350° C. C then had a nickel content (expressed as CuO) of 4% by weight.

[0040] At the same time, three prior art catalysts termed D, E and Fwere selected; they were in the form of cylindrical extrudates. D was acatalyst based on titanium oxide and doped with chromium oxide, but didnot contain any sulphates. E and F were alumina-based catalysts.

[0041] The compositions and specific surface areas of catalysts A to Fare shown in Table 1. TABLE 1 characteristics of study catalystsCatalyst A B C D E F TiO₂ (%) 88.0 86.2 84.5 90.0 — — Al₂O₃ (%) — — — —80 80 CaSO₄ (%) 11.0 10.8 10.6 — — — NiO (%) — 2.1 — — — 3.1 CuO (%) — —4.0 — — — CoO (%) — — — — 3.4 — MoO₃ (%) — — — — 14.2 14.5 Cr₂O₃ (%) — —— 6.2 — — specific 116 105 101 72 177 191 surface area (m²/g) diameter3.5 3.5 3.5 3.5 1.6 1.6 (mm)

[0042] The results obtained using these various catalysts was thenstudied during the treatment of a gas with the following composition,representative of that which can be found in gas from a co-generationinstallation (all percentages are given by volume):

[0043] 30% to 40% for CO and H₂;

[0044] 2% to 18% for H₂O;

[0045] 0 to 2000 ppm for COS, with a H₂S concentration about ten timesthat of COS but never less than 2000 ppm;

[0046] 0 to 500 ppm for HCN;

[0047] 0 to 1000 ppm for NH₃;

[0048] 0 to 150 ppm for HCl.

[0049] The temperature of the gas was fixed at between 180° C. and 280°C., and their pressure was between 1 and 10 bars. The hourly spacevelocity (HSV, the ratio between the weight of feed treated per unittime to the weight of catalyst used) was fixed between 2950 and 5900h⁻¹.

EXAMPLE 1

[0050] A first series of experiments was conducted in the absence ofHCN, and also in the absence of NH₃ and HCl, the concentration of COS atthe reactor inlet being 2000 ppm.

[0051] With the temperature at 220° C., the pressure at 1 bar and thewater content at the reactor inlet at 8% with a HSV of 5900 h⁻¹, the COSconversions obtained with catalysts A, B, C, D, E and F were 95.5%,97.5%, 96.2%, 78.5%, 56.6% and 57.4% respectively.

[0052] With the temperature at 210° C., the pressure at 1 bar and thewater content at the reactor inlet at 18% with a HSV of 5900 h⁻¹, theCOS conversions obtained with catalysts A, D and E were 98.2%, 72.4% and52.1% respectively.

EXAMPLE 2

[0053] A second series of experiments was conducted in the presence of500 ppm of HCN, but in the absence of NH₃ and HCl, the concentration ofCOS at the reactor inlet being 2000 ppm.

[0054] With the temperature at 220° C., the pressure at 1 bar and thewater content at the reactor inlet at 8% with a HSV of 5900 h-1, the COSconversions obtained with catalysts A, B, C, D, E and F were 85.8%,90.5%, 90.2%, 68.5%, 40.2% and 41.8% respectively. At the same time, theHCN conversions obtained with the same catalysts were 95.5%, 98.2%,97.1%, 96.0%, 85.2% and 81.3% respectively. At the same time, theextraneous production of CO₂ via CO shift conversion, was 0.15%, 0.2%,0.2%, 1.1%, 1.4% and 2.3% respectively, the temperature increase wasless than 1° C. for catalysts A, B and C, but 7° C., 10° C. and 15° C.for catalysts D, E and F. Further, 10%, 6% and 15% of the transformedHCN was in fact hydrogenated to CH₄ with D, E and F respectively,wherein less than 1% was transformed with A, B and C.

[0055] With the temperature at 220° C., the pressure at 1 bar and thewater content at the reactor inlet at 15% with a HSV of 5900 h⁻¹, theCOS conversions obtained with catalysts A, D, E and F were 94.0%, 78.4%,50.4% and 48.7% respectively. The respective HCN conversions obtainedwith these same four catalysts were 95.7%, 95.5%, 88.6% and 84.9%. Atthe same time, the extraneous production of CO₂ via CO shift conversion,was 0.15%, 0.7%, 3.3% and 3.1% by volume respectively, the temperatureincrease being less than 1° C. for catalyst A, but 5° C., 17° C. and 17°C. for catalysts D, E and F. The remarks made in Example 1 regardingmethane formation are also applicable in this example.

[0056] With the temperature at 180° C., the pressure at 10 bars and thewater content at the reactor inlet at 6% with a HSV of 2950 h-1, the COSconversions obtained with catalysts A and B were 94.6% and 97.1%respectively. At the same time, the HCN conversions obtained with thesame catalysts were 90.8% and 93.7% respectively. No significantformation of CO₂, CH₄ or any particular temperature rise was observed.

EXAMPLE 3

[0057] A third series of experiments was conducted in the presence of500 ppm of HCN and 2000 ppm of NH₃, the concentration of COS at thereactor inlet being 2000 ppm.

[0058] With the temperature at 220° C., the pressure at 1 bar and thewater content at the reactor inlet at 15% with a HSV of 5900 h-1, theCOS conversions obtained with catalysts A, D and E were 94.1%, 74.4% and41.4% respectively. At the same time, the HCN conversions obtained withthe same three catalysts were 95.8%, 91.5% and 78.4% respectively.

EXAMPLE 4

[0059] A fourth series of experiments was conducted in the presence of500 ppm of HCN and 150 ppm of HCl, the concentration of COS at thereactor inlet being 2000 ppm.

[0060] With the temperature at 220° C., the pressure at 1 bar and thewater content at the reactor inlet at 8% with a HSV of 5900 h⁻¹, the COSconversions obtained with catalysts A, D and E were 70.6%, 58.4% and25.9% respectively. At the same time, the HCN conversions obtained withthe same three catalysts were 90.5%, 51.0% and 30.7% respectively. Whenthe supply of HCl to the reactor was cut off, the rest of the conditionsremaining unchanged, the performance of A in COS hydrolysis slowlyreturned to normal, contrary to that of D which only partially recoveredits initial level, while E had been visibly damaged.

[0061] From these observations, it can be seen that the catalysts of theinvention present an optimum compromise between very high conversion ofCOS and HCN, insensitivity to the presence of NH₃, resistance to andreversibility on exposure to HCl for COS conversion (that of HCN beingunaffected by HCl), and a remarkable limitation to the formation of CO₂and CH₄.

[0062] The prior art catalysts, in contrast, had substantially lowerconversions than with COS and usually for HCN and all caused theformation of undesirable compounds, as well as increased extraneousexothermicity. Further, exposure to by-products that couldconventionally be encountered (NH₃, HCl), was difficult to accommodateand even caused severe damage to catalytic performance.

1. Use of a composition based on TiO₂ as a catalyst for hydrolyzing COS′and/or HCN in a gas mixture, said composition comprising at least 1% byweight, preferably at least 5%, of at least one sulphate of analkaline-earth metal selected from calcium, barium, strontium andmagnesium.
 2. Use according to claim 1, characterized in that thecomposition comprises at least 40% by weight of TiO₂, preferably atleast 60%.
 3. Use according to claim 1, characterized in that saidsulphate is calcium sulphate.
 4. Use according to claim 1, characterizedin that the composition also comprises at least one compound selectedfrom clays, silicates, titanium sulphate and ceramic fibres in a totalcontent of 30% by weight or less, preferably in the range 0.5% to 15%.5. Use according to claim 1, characterized in that the compositioncomprises at least 60% by weight of TiO₂, at least 0.1% by weight and atmost 20% by weight, advantageously at most 15%, preferably at most 10%,of a doping compound or a combination of doping compounds selected fromcompounds of iron, vanadium, cobalt, nickel, copper, molybdenum andtungsten.
 6. Use according to claim 5, characterized in that the dopingcompound or compounds is/are oxides.
 7. Use according to claim 1,characterized in that the catalyst has been formed by extrusion.
 8. Useaccording to claim 7, characterized in that the transverse section ofthe catalyst is in the range 0.5 to 8 mm, preferably in the range 0.8 to5 mm.
 9. Use according to claim 1, characterized in that said gasmixture is derived from a co-generation installation.